Organic material for non-linear optics

ABSTRACT

An organic material for non-linear optics comprising: 
     an electron donor group constituted by a heteroaromatic cycle; 
     an electron attractor group selected from NO 2  and CN; and 
     an electron effect transmitter group selected from an aromatic system of the phenyl ring type, or a chain having n atoms of carbon including a double or a triple bond (n&lt;4).

The present invention relates to an organic material for non-linearoptics.

Materials having non-linear properties in optics may be used fornumerous applications: frequency doublers; optical bistables; switches;modulators; directional couplers; parametric amplifiers; etc.

A small number of inorganic materials which are non-isotropic, and inparticular non-centrosymmetrical, are known which possess a non-zerosecond order coefficient and non-linear optical properties. Theseinclude in particular potassium-dihydrogen-phosphate (KDP), and lithiumniobate and tantalate (LiNbO₃, LiTaO₃, . . . ).

It is often difficult to make these inorganic materials. That is whyinterest has been shown in organic materials having non-linearproperties, in particular when the materials are polymers,thermoplastic, or film-forming.

Optical non-linearity is generally obtained by incorporating highlyhyperpolarizable polar molecules or groups of molecules in the organicmaterial by mixing or by chemical grafting.

Such molecules are synthesized by associating an electron attractorgroup with an electron donor group via a group for transmitting electroneffects.

For example, the following materials are known:

paranitroaniline (PAN) ##STR1## N (nitro 4 phenyl) N methyl amino 2acetonitrile (NPAN) ##STR2## dinitro 2-4 phenyl L alanine (MAP) ##STR3##methyl 3 nitro 4 pyridine N-oxide (POM) ##STR4##

These molecules are characterized by their coefficient ofhyperpolarizability β defined by the series development of the dipolarmoment μ as a function of the electric field E

    μ=μ.sub.o +α.E+β|E|.sup.2 + . . .

The coefficient β may be measured by the EFISH method (Electric FieldInduced Second Harmonic generation).

Many authors judge the value of a synthesized molecule on the basis ofthe value of its molecular hyperpolarizability associated with infraredfrequency doubling.

For these applications, molecules having a large number of atoms may besuitable, however experience shows that such molecules absorb lightstrongly in the visible spectrum and this may be unacceptable. Forelectro-optical applications, the useful magnitude is ratherhyperpolarizability per unit mass or per unit volume.

The object of the present invention is to find molecules having all theessential parts (attractor group, donor group, and transmitter group)using as small a number of atoms as possible, while neverthelessconserving a good value of constant field molecular hyperpolarizability.

The present invention provides an organic material for non-linear opticscomprising an electron attractor group and an electron donor groupinterconnected by an electron effect transmitter group, the materialbeing characterized by the fact that:

the electron donor group comprises an aromatic heterocycle in the formof a 5 member ring that includes a heteroatom selected from S, O, and N;

the electron attractor group is selected from NO₂ and CN; and

the transmitter group is selected from the group consisting of carbonchains having n atoms of carbon (where n<4) and including a double or atriple bond, and an aromatic system such as a phenyl ring.

The nitrogen atom may be bonded to a radical R selected from H, CH₃, andC₂ H₅.

In a first variant, the said transmitter group is constituted by anethenyl bond.

By way of example, the material of the invention may satisfy one of thefollowing formulas: ##STR5##

In a second variant, the said transmitter group is constituted by aphenyl ring. The material of the invention may then satisfy one of thefollowing formulas: ##STR6##

In another variant, the said heteroaromatic cycle is substituted at 5 bya group selected from CH₃ O, CH₃ S, NH₂, CH₃ NH, (CH₃)₂ N.

In all molecules of the invention, the electron donor characteristic andthe aromatic characteristic are concentrated on a smaller number ofatoms than in prior art molecules since they contain aromatic cycleswhich are rich in electrons (substituted or non-substituted furan,thiophene, or pyrrole).

With these molecules possessing a small number of atoms, it is possibleto achieve high molecular hyperpolarizability at zero frequency.

Excellent results may be obtained by using the cyano group (--CN) whichis smaller than the nitro group (--NO₂).

The present invention also provides methods of synthesizing the abovematerials.

Other characteristics and advantages of the present invention appearfrom the following description of embodiments given by way ofnon-limiting illustration.

EXAMPLE 1

(Transnitrovinylidene) - 2 thiophene was prepared: ##STR7##

In a 250 cm³ three-necked Wolff bottle fitted with a thermometer, adropping funnel, and a septum, the following were placed: 5.60 grams (g)of thiophene - 2 carbaldehyde; 3.55 g of nitromethane; 1.01 g of Aliquat336 (Aldrich); and 100 ml of methanol. The mixture was cooled to about0° C. to 5° C.

Under magnetic stirring, 10 ml of 10N caustic soda were addeddrop-by-drop using the 10 ml dropping funnel while ensuring that thetemperature did not rise above 5° C.

Stirring was continued for 10 minutes after all the soda had been added.Then the entire mixture was transferred drop-by-drop into 50 ml of asolution of 4N hydrochloric acid.

The mixture was then stored at 4° C. for 24 hours. Crystals were formedwhich were filtered and washed in distilled water and then dried for 36hours.

This provided 6.9 g of product, i.e. a yield of 89%. The product wasthen purified by filtering on silica.

EXAMPLE 2

(Transnitrovinylidene) - 3 thiophene was prepared. ##STR8##

The above procedure was performed using 5.4 g of thiophene - 3carbaldehyde. 6.6 g of product were obtained, i.e. a yield of 85%.

EXAMPLE 3

(Transnitrovinylidene) - 2 furan was prepared ##STR9##

The above procedure was applied to 4.8 g of furfuraldehyde. 5.02 g ofproduct were obtained, i.e. a yield of 72%.

EXAMPLE 4

(Transnitrovinylidene) - 3 furan was prepared ##STR10##

The above procedure was applied to 4.8 g of furan -3 carbaldehyde. 5.7 gof product were obtained, i.e. a yield of 82%.

The four materials analogous to the above four materials but in whichthe nitro group is replaced by a cyano group may be prepared in similarmanner.

EXAMPLE 5

(Cyano 4 phenyl) - 2 furan was prepared ##STR11##

Since bromo-2-furan is not commercially available, the correspondingzinc derivative was prepared by treating 5 m.moles of furan with 6m.moles of butyllithium in 50 ml of tetrahydrofuran (THF). The resultingsolution was then added to a solution of 6 m.moles of zinc chlorideZnCl₂ in 60 ml of THF. The resulting zinc compound was then made toreact on 5 m.moles of parabromobenzonitrile in the presence of thepalladium complex Pd[P(C₆ H₅)₃ ]₄. (Cyano 4 phenyl) - 2 furan wasobtained with a yield of 74% of isolated product.

The absorption of this material was at a maximum at 304 nanometers, withits molecular absorption coefficient ε then being 48,740. Thiscoefficient was close to 0 at 400 nanometers. Its optical transparencywas perfect in the frequency range of the visible spectrum and the nearinfrared. It may therefore be used for generating a second harmonic inthe blue region of the visible spectrum using a laser in the infrared orthe near infrared.

EXAMPLE 6

(Cyano 4 phenyl) - 2 thiophene was prepared ##STR12##

This was done by preparing the magnesium derivative of bromo 2 thiophene(5 m.moles) in 50 ml of THF. The magnesium derivative was caused toreact with paraiodobenzonitrile in the presence of the palladium complexPd[P(C₆ H₅)₃ ]₄.

(Cyano 4 - phenyl) - 2 thiophene was obtained with a yield of 73% forthe isolated product.

This material had an absorption maximum at 302 nanometers, where itsmolecular absorption coefficient ε was 87,623. This coefficient wasclosed to 0 at 400 nanometers. It has the same advantages of the productof Example 5.

EXAMPLE 7

(Nitro 4 phenyl) - 2 furan may be obtained in analogous manner from##STR13##

Its absorption maximum was situated at 342 nanometers with an absorptioncoefficient ε of 16,666, which coefficient reduces to 1935 at 400nanometers.

EXAMPLE 8

Similarly, (nitro 4 phenyl) - 2 thiophene may be obtained from ##STR14##

Its absorption maximum was at 344 nanometers with an absorptioncoefficient ε of 15,537. This coefficient reduces to 1624 at 400nanometers.

Examples 5 to 8 how that excellent results can be obtained using a cyanoattractor group (--CN) which is smaller than a nitro group (--NO₂).

The use of cyano derivatives turns out to be particularly advantageousin visible light since these products absorb this light much less thando the corresponding nitrate derivatives.

Naturally, the invention is not limited to the specific examplesdescribed, nor to the methods of synthesis mentioned.

We claim:
 1. An organic material having non-linear optical properties,characterized by the fact that it satisfied one of the followingformulas: ##STR15##
 2. An organic material having non-linear opticalproperties, characterized by the fact that it satisfies one of thefollowing formulas: ##STR16##
 3. An organic material having non-linearoptical properties, characterized by the fact that it satisfies thefollowing formula: ##STR17##
 4. An organic material having non-linearproperties, characterized by the fact that it satisfies the followingformula: ##STR18##
 5. A method of preparing a material that satisfiesthe formula ##STR19## comprising coupling a zinc derivative having theformula ##STR20## with para-bromobenzonitrile in the presence ofcatalytic quantities of palladium complexes.
 6. A method of preparing amaterial that satisfies the formula ##STR21## comprising coupling amagnesium derivative having the formula ##STR22## withpara-iodobenzonitrile in the presence of catalytic quantities ofpalladium complexes.
 7. A method of preparing a material that satisfiesthe formula ##STR23## comprising coupling a zinc derivative having theformula ##STR24## with para-iodonitrobenzene in the presence ofcatalytic quantities of palladium complexes.
 8. A method of preparing amaterial that satisfies the formula ##STR25## comprising coupling a zincderivative having the formula ##STR26## with para-iodonitrobenzene inthe presence of catalytic quantities of palladium complexes.